(Temporary?) fix? Copied FCP_SMT_232_HSSE_Flux_Presets.vstsound to where I store my Halion Sonic SE libraries. Double-clicked, registered it with the Halion Library Manager, restarted Cubase, and they show up.

The .vstsound is in the update files at Steinberg\WIN64\Cubase Pro 9.5 - Update from 9.0\Cubase 9.5\Cubase for Windows\Additional Content\VST Sound\PresetVSTSound

How did you choose to install Cubase? For the current user or for all users?
Unfortunately Cubase installer never uses the choice which was initially made, which can end up in a folder mess with every update if you do not remember your first choice and then may change from Every / All Users to the current.
I found now the Flux vstsound file in my profile path but somehow it cannot be loaded or accessed. It seems as a bug like HSE does not like the Roaming profile path

If you have installed the free HALion Sonic SE 3 before, the Cubase 9.5 Installer will not install HALion Sonic SE 3 again and also not the new Flux Synthesizer.
Like already mentioned above, running the Cubase 9.5 Installer again will allow you to reinstall HALion Sonic SE 3 and will also install Flux.

Well, I tried that already and do not get Flux in the right way.
Or, how is Flux supposed to be opened?
I can see it sometimes when I click the preset browser from within Halion SE. But it then shows as greyed out and when I try to highlight it, it vanishes and does not show up again.
I had indeed HSE 3 installed before. So I removed HSE & content completely and run the full installer of CB 9.5 Pro and it showed HSE & content to be reinstalled. Afterwards no difference. Flux is fluctoating...

Thanks to everyone here, and especially judemiller, I finally got my Flux working. The first part of the problem though is that Flux is not even in the full 11GB installer. I just looked in the Cubase for Windows\Additional Content\VST Sound\PresetVSTSound of the full installer and there is nothing inside. I then downloaded just the 1.43GB update and in that folder was the FCP_SMT_232_HSSE_Flux_Presets.vstsound .
So, as suggested by judemiller I then copied FCP_SMT_232_HSSE_Flux_Presets.vstsound to Halion Sonic SE libraries directory, double-clicked to registered it with the Halion Library Manager, restarted Cubase, and now it works.
So for those of you who are not even finding the files, download the update version and try again because that's where it is. Not in the full version. I guess Steinberg will address this in time. Cheers.